This invention relates generally to computed tomography (CT) imaging and more particularly, to generating images of a heart.
In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In at least one known type of imaging system, commonly known as a computer tomography (CT) system, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a xe2x80x9chelicalxe2x80x9d scan may be performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reduced scanning time, helical scanning provides other advantages such as improved image quality and better control of contrast.
In helical scanning, and as explained above, only one view of data is collected at each slice location. To reconstruct an image of a slice, the other view data for the slice is generated based on the data collected for other views. Helical reconstruction algorithms are known, and described, for example, in C. Crawford and K. King, xe2x80x9cComputed Tomography Scanning with Simultaneous Patient Translation,xe2x80x9d Med. Phys. 17(6), November/December 1990.
In order to generate images of a rapidly moving object, such as a heart, known imaging systems have minimized motion artifacts, caused by the movement of the heart, by utilizing a high rotational speed gantry or by incorporating electron beam technology. However, the high speed gantry system significantly increases the force applied to the x-ray source and the detector affecting performance of the system. The electron beam technology requires a very complex design that significantly increases the cost of the scanner. As a result, few systems are capable of generating images of a moving heart without generating images containing significant motion artifacts.
To generate images of a heart, it is desirable to provide an imaging system which gathers data at a sufficiently high rate so that heart motion artifacts are minimized. It would also be desirable to provide such a system which generates such images of the entire heart using a single scan.
These and other objects may be attained by a cardiac CT scanner that generates images of an entire object of interest using projection data collected from a plurality of detector arrays rotated less than a full rotation around the object. In accordance with one embodiment of the present invention, a plurality of angularly spaced source-detector array pairs are utilized to generate projection data of a limited area, or field of view, containing only the object of interest, so that motion artifacts are minimized. More specifically and in one embodiment, as a result of the quantity and spacing of the pairs, sufficient projection data is collected in less than one full rotation of the gantry to generate images of a heart. Particularly, sufficient projection data to generate an image of the entire heart is collected in significantly less than one cardiac cycle. As a result, motion artifacts are minimized in the reconstructed images.
In one embodiment, two source-detector pairs are utilized to generate images of the entire heart without significant motion artifacts. By collecting projection data for a limited field of view containing only the heart, the size of the detector array is reduced and minimized motion artifact images are generated by collecting xcfx80 plus a fan angle, which is the angle of the beam projected from the x-ray source, of projection data. More specifically, where the pairs are offset with respect to each other by an angle, xcex2, equal to the quantity of (xcfx80+fan angle)/the number of pairs, so that little overlapped data is collected, the images may be generated by rotating the gantry (xcfx80+fan angle)/the number of pairs degrees.
Particularly to generate images of the heart, initially the patient is positioned so that the patient""s heart is centered at an iso-center of the system, for example by performing one or more scout scans. A scan is then completed by enabling x-ray sources of each of the source-detector pairs, and collecting projection data as the gantry is rotated. The projection data, collected during a period significantly less than a typical cardiac cycle, is then reconstructed. As a result of the timing of the data collection, motion artifacts are virtually eliminated from the reconstructed images of the heart.
In another embodiment, the size of the detector arrays of each source-detector array pair may be further reduced by generating images of the entire heart by performing at least two separate scan of the heart. For example, by performing two scans, the number of rows of each detector is reduced by one-half. The projection data from each scan are then combined to generate an image of the entire heart.
In another embodiment, each source-detector pair source has at least two focal spots. By properly selecting the spacing between the focal spots, images of the same heart volume are collected using a further reduced size detector.
The above described system generates images of the heart by gathering data at a sufficiently high rate so that heart motion artifacts are minimized. In addition, the system generates such images of the entire heart using a single scan.